> I have seen electronic scales use current to electromagnetically counter
> gravity, with the equilbrium point being determined by light that was
> obstructed by a blade on the moving part.  The current servos until the
> light is dimmed to about half.  Over narrow ranges the
> response was quite linear.  But I would think that force transducers
> would be a more rugged approach.

Interesting notion.  How about a mechanical balance with a weight that slides
along a lever under software control via either stepper or motor+quad encoder?
Should be simple enough to program, and the mechanics shouldn't be too bad
either.